The invention resides in a pH-sensitive microsensor on silicon basis having a sensor membrane and in a method of making the microsensor.
The publications by T. Matsuro et al., "METHODS OF ISFET FABRICATION", Sensors and Actuators 1 (1981) p. 77, H. Abe et al., "ISFETs Using Inorganic Gate Thin Films", IEEE Trans. Electron. Dev. Ed. 26, 12 (1979, page 1939 or Bousse et al., Hysterisis in Al.sub.2 O.sub.3 -gate ISFETS, Sensors and Actuators B2 (1990) page 103, disclose silicon-based pH-sensitive microsensors.
These are so-called ion sensitive field effect transistors (ISFET), which, in their basic construction, correspond to a MOSFET. In place of the metallic gate electrode such a component includes a combination of a sensitive layer (sensor membrane), electrolyte and reference electrode. Furthermore, C. Cin et al., "An experimental study of inorganic gate ISFETS and Actuators B,1 (1990), p. 421, discloses that, instead of a complete ISFET, it is possible for example to use as pH-sensors also capacitive field effect structures, which, in their construction, correspond to the gate area of an ISFET.
In a pH-sensitive ISFET, this sensor membrane consists for example of Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5 or also Al.sub.2 O.sub.3. The best results with respect to sensitivity, stability, and selectivity were obtained for Ta.sub.2 O.sub.5 and Al.sub.2 O.sub.3.
The sensor membrane, particularly if AL.sub.2 O.sub.3 is selected as the membrane material, is formed by sputter technology or by Chemical Vapor Deposition (CVD).
Microsensors, which include such sensor membranes have the disadvantage that they have relatively high drift rates.
It is therefore the object of the present invention to provide a sensor and a method of manufacturing such a sensor wherein the drift rate is reduced.